Rollup merge of #144560 - Zalathar:auto-derived, r=compiler-errors

coverage: Treat `#[automatically_derived]` as `#[coverage(off)]`

One of the contributing factors behind https://github.com/rust-lang/rust/issues/141577#issuecomment-3120667286 was the presence of derive-macro-generated code containing nested closures.

Coverage instrumentation already has a heuristic for skipping code marked with `#[automatically_derived]` (rust-lang/rust#120185), because derived code is usually not worth instrumenting, and also has a tendency to trigger vexing edge-case bugs in coverage instrumentation or coverage codegen.

However, the existing heuristic only applied to the associated items directly within an auto-derived impl block, and had no effect on closures or nested items within those associated items.

This PR therefore extends the search for `#[coverage(..)]` attributes to also treat `#[automatically_derived]` as an implied `#[coverage(off)]` for the purposes of coverage instrumentation.

---

This change doesn’t rule out an entire category of bugs, because it only affects code that actually uses the auto-derived attribute. But it should reduce the overall chance of edge-case macro span bugs being observed in the wild.

10 files changed, 396 insertions, 42 deletions

diff --git a/compiler/rustc_attr_data_structures/src/attributes.rs b/compiler/rustc_attr_data_structures/src/attributes.rs
index 610c93a253c..55019cd57a7 100644
--- a/compiler/rustc_attr_data_structures/src/attributes.rs
+++ b/compiler/rustc_attr_data_structures/src/attributes.rs
@@ -110,18 +110,10 @@ pub enum DeprecatedSince {
     Err,
 }
 
-#[derive(
-    Copy,
-    Debug,
-    Eq,
-    PartialEq,
-    Encodable,
-    Decodable,
-    Clone,
-    HashStable_Generic,
-    PrintAttribute
-)]
-pub enum CoverageStatus {
+/// Successfully-parsed value of a `#[coverage(..)]` attribute.
+#[derive(Copy, Debug, Eq, PartialEq, Encodable, Decodable, Clone)]
+#[derive(HashStable_Generic, PrintAttribute)]
+pub enum CoverageAttrKind {
     On,
     Off,
 }
@@ -304,8 +296,8 @@ pub enum AttributeKind {
     /// Represents `#[const_trait]`.
     ConstTrait(Span),
 
-    /// Represents `#[coverage]`.
-    Coverage(Span, CoverageStatus),
+    /// Represents `#[coverage(..)]`.
+    Coverage(Span, CoverageAttrKind),
 
     ///Represents `#[rustc_deny_explicit_impl]`.
     DenyExplicitImpl(Span),
diff --git a/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs b/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs
index bb28121c2c5..afa9abed0bb 100644
--- a/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs
@@ -1,4 +1,4 @@
-use rustc_attr_data_structures::{AttributeKind, CoverageStatus, OptimizeAttr, UsedBy};
+use rustc_attr_data_structures::{AttributeKind, CoverageAttrKind, OptimizeAttr, UsedBy};
 use rustc_feature::{AttributeTemplate, template};
 use rustc_session::parse::feature_err;
 use rustc_span::{Span, Symbol, sym};
@@ -78,16 +78,16 @@ impl<S: Stage> SingleAttributeParser<S> for CoverageParser {
             return None;
         };
 
-        let status = match arg.path().word_sym() {
-            Some(sym::off) => CoverageStatus::Off,
-            Some(sym::on) => CoverageStatus::On,
+        let kind = match arg.path().word_sym() {
+            Some(sym::off) => CoverageAttrKind::Off,
+            Some(sym::on) => CoverageAttrKind::On,
             None | Some(_) => {
                 fail_incorrect_argument(arg.span());
                 return None;
             }
         };
 
-        Some(AttributeKind::Coverage(cx.attr_span, status))
+        Some(AttributeKind::Coverage(cx.attr_span, kind))
     }
 }
 
diff --git a/compiler/rustc_mir_transform/src/coverage/query.rs b/compiler/rustc_mir_transform/src/coverage/query.rs
index 551f720c869..73795e47d2e 100644
--- a/compiler/rustc_mir_transform/src/coverage/query.rs
+++ b/compiler/rustc_mir_transform/src/coverage/query.rs
@@ -1,4 +1,4 @@
-use rustc_attr_data_structures::{AttributeKind, CoverageStatus, find_attr};
+use rustc_attr_data_structures::{AttributeKind, CoverageAttrKind, find_attr};
 use rustc_index::bit_set::DenseBitSet;
 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::coverage::{BasicCoverageBlock, CoverageIdsInfo, CoverageKind, MappingKind};
@@ -32,16 +32,6 @@ fn is_eligible_for_coverage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
         return false;
     }
 
-    // Don't instrument functions with `#[automatically_derived]` on their
-    // enclosing impl block, on the assumption that most users won't care about
-    // coverage for derived impls.
-    if let Some(impl_of) = tcx.impl_of_assoc(def_id.to_def_id())
-        && tcx.is_automatically_derived(impl_of)
-    {
-        trace!("InstrumentCoverage skipped for {def_id:?} (automatically derived)");
-        return false;
-    }
-
     if tcx.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::NAKED) {
         trace!("InstrumentCoverage skipped for {def_id:?} (`#[naked]`)");
         return false;
@@ -57,20 +47,32 @@ fn is_eligible_for_coverage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
 
 /// Query implementation for `coverage_attr_on`.
 fn coverage_attr_on(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
-    // Check for annotations directly on this def.
-    if let Some(coverage_status) =
-        find_attr!(tcx.get_all_attrs(def_id), AttributeKind::Coverage(_, status) => status)
+    // Check for a `#[coverage(..)]` attribute on this def.
+    if let Some(kind) =
+        find_attr!(tcx.get_all_attrs(def_id), AttributeKind::Coverage(_sp, kind) => kind)
     {
-        *coverage_status == CoverageStatus::On
-    } else {
-        match tcx.opt_local_parent(def_id) {
-            // Check the parent def (and so on recursively) until we find an
-            // enclosing attribute or reach the crate root.
-            Some(parent) => tcx.coverage_attr_on(parent),
-            // We reached the crate root without seeing a coverage attribute, so
-            // allow coverage instrumentation by default.
-            None => true,
+        match kind {
+            CoverageAttrKind::On => return true,
+            CoverageAttrKind::Off => return false,
         }
+    };
+
+    // Treat `#[automatically_derived]` as an implied `#[coverage(off)]`, on
+    // the assumption that most users won't want coverage for derived impls.
+    //
+    // This affects not just the associated items of an impl block, but also
+    // any closures and other nested functions within those associated items.
+    if tcx.is_automatically_derived(def_id.to_def_id()) {
+        return false;
+    }
+
+    // Check the parent def (and so on recursively) until we find an
+    // enclosing attribute or reach the crate root.
+    match tcx.opt_local_parent(def_id) {
+        Some(parent) => tcx.coverage_attr_on(parent),
+        // We reached the crate root without seeing a coverage attribute, so
+        // allow coverage instrumentation by default.
+        None => true,
     }
 }
 
diff --git a/tests/coverage/auto-derived.auto.cov-map b/tests/coverage/auto-derived.auto.cov-map
new file mode 100644
index 00000000000..e3d411d895a
--- /dev/null
+++ b/tests/coverage/auto-derived.auto.cov-map
@@ -0,0 +1,23 @@
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::inner_fn_on
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 1f, 09, 00, 19, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 23, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 31, 9) to (start + 0, 25)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 35)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: auto_derived::main
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 33, 01, 00, 0a, 01, 01, 05, 00, 1a, 01, 01, 01, 00, 02]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 3
+- Code(Counter(0)) at (prev + 51, 1) to (start + 0, 10)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 26)
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
+Highest counter ID seen: c0
+
diff --git a/tests/coverage/auto-derived.auto.coverage b/tests/coverage/auto-derived.auto.coverage
new file mode 100644
index 00000000000..242abbf8031
--- /dev/null
+++ b/tests/coverage/auto-derived.auto.coverage
@@ -0,0 +1,54 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |//@ edition: 2024
+   LL|       |//@ revisions: base auto on
+   LL|       |
+   LL|       |// Tests for how `#[automatically_derived]` affects coverage instrumentation.
+   LL|       |//
+   LL|       |// The actual behaviour is an implementation detail, so this test mostly exists
+   LL|       |// to show when that behaviour has been accidentally or deliberately changed.
+   LL|       |//
+   LL|       |// Revision guide:
+   LL|       |// - base: Test baseline instrumentation behaviour without `#[automatically_derived]`
+   LL|       |// - auto: Test how `#[automatically_derived]` affects instrumentation
+   LL|       |// - on:   Test interaction between auto-derived and `#[coverage(on)]`
+   LL|       |
+   LL|       |struct MyStruct;
+   LL|       |
+   LL|       |trait MyTrait {
+   LL|       |    fn my_assoc_fn();
+   LL|       |}
+   LL|       |
+   LL|       |#[cfg_attr(auto, automatically_derived)]
+   LL|       |#[cfg_attr(on, automatically_derived)]
+   LL|       |#[cfg_attr(on, coverage(on))]
+   LL|       |impl MyTrait for MyStruct {
+   LL|       |    fn my_assoc_fn() {
+   LL|       |        fn inner_fn() {
+   LL|       |            say("in inner fn");
+   LL|       |        }
+   LL|       |
+   LL|       |        #[coverage(on)]
+   LL|      1|        fn inner_fn_on() {
+   LL|      1|            say("in inner fn (on)");
+   LL|      1|        }
+   LL|       |
+   LL|       |        let closure = || {
+   LL|       |            say("in closure");
+   LL|       |        };
+   LL|       |
+   LL|       |        closure();
+   LL|       |        inner_fn();
+   LL|       |        inner_fn_on();
+   LL|       |    }
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |#[inline(never)]
+   LL|       |fn say(s: &str) {
+   LL|       |    println!("{s}");
+   LL|       |}
+   LL|       |
+   LL|      1|fn main() {
+   LL|      1|    MyStruct::my_assoc_fn();
+   LL|      1|}
+
diff --git a/tests/coverage/auto-derived.base.cov-map b/tests/coverage/auto-derived.base.cov-map
new file mode 100644
index 00000000000..2dcd616300d
--- /dev/null
+++ b/tests/coverage/auto-derived.base.cov-map
@@ -0,0 +1,61 @@
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn
+Raw bytes (34): 0x[01, 01, 00, 06, 01, 19, 05, 00, 15, 01, 0a, 0d, 00, 14, 01, 04, 09, 00, 12, 01, 01, 09, 00, 11, 01, 01, 09, 00, 14, 01, 01, 05, 00, 06]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 6
+- Code(Counter(0)) at (prev + 25, 5) to (start + 0, 21)
+- Code(Counter(0)) at (prev + 10, 13) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 4, 9) to (start + 0, 18)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 17)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 6)
+Highest counter ID seen: c0
+
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::inner_fn
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 1a, 09, 00, 16, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 1e, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 26, 9) to (start + 0, 22)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 30)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::inner_fn_on
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 1f, 09, 00, 19, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 23, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 31, 9) to (start + 0, 25)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 35)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::{closure#0}
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 23, 1a, 00, 1b, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 1d, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 35, 26) to (start + 0, 27)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 29)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: auto_derived::main
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 33, 01, 00, 0a, 01, 01, 05, 00, 1a, 01, 01, 01, 00, 02]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 3
+- Code(Counter(0)) at (prev + 51, 1) to (start + 0, 10)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 26)
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
+Highest counter ID seen: c0
+
diff --git a/tests/coverage/auto-derived.base.coverage b/tests/coverage/auto-derived.base.coverage
new file mode 100644
index 00000000000..6ab6aa5c4df
--- /dev/null
+++ b/tests/coverage/auto-derived.base.coverage
@@ -0,0 +1,54 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |//@ edition: 2024
+   LL|       |//@ revisions: base auto on
+   LL|       |
+   LL|       |// Tests for how `#[automatically_derived]` affects coverage instrumentation.
+   LL|       |//
+   LL|       |// The actual behaviour is an implementation detail, so this test mostly exists
+   LL|       |// to show when that behaviour has been accidentally or deliberately changed.
+   LL|       |//
+   LL|       |// Revision guide:
+   LL|       |// - base: Test baseline instrumentation behaviour without `#[automatically_derived]`
+   LL|       |// - auto: Test how `#[automatically_derived]` affects instrumentation
+   LL|       |// - on:   Test interaction between auto-derived and `#[coverage(on)]`
+   LL|       |
+   LL|       |struct MyStruct;
+   LL|       |
+   LL|       |trait MyTrait {
+   LL|       |    fn my_assoc_fn();
+   LL|       |}
+   LL|       |
+   LL|       |#[cfg_attr(auto, automatically_derived)]
+   LL|       |#[cfg_attr(on, automatically_derived)]
+   LL|       |#[cfg_attr(on, coverage(on))]
+   LL|       |impl MyTrait for MyStruct {
+   LL|      1|    fn my_assoc_fn() {
+   LL|      1|        fn inner_fn() {
+   LL|      1|            say("in inner fn");
+   LL|      1|        }
+   LL|       |
+   LL|       |        #[coverage(on)]
+   LL|      1|        fn inner_fn_on() {
+   LL|      1|            say("in inner fn (on)");
+   LL|      1|        }
+   LL|       |
+   LL|      1|        let closure = || {
+   LL|      1|            say("in closure");
+   LL|      1|        };
+   LL|       |
+   LL|      1|        closure();
+   LL|      1|        inner_fn();
+   LL|      1|        inner_fn_on();
+   LL|      1|    }
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |#[inline(never)]
+   LL|       |fn say(s: &str) {
+   LL|       |    println!("{s}");
+   LL|       |}
+   LL|       |
+   LL|      1|fn main() {
+   LL|      1|    MyStruct::my_assoc_fn();
+   LL|      1|}
+
diff --git a/tests/coverage/auto-derived.on.cov-map b/tests/coverage/auto-derived.on.cov-map
new file mode 100644
index 00000000000..2dcd616300d
--- /dev/null
+++ b/tests/coverage/auto-derived.on.cov-map
@@ -0,0 +1,61 @@
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn
+Raw bytes (34): 0x[01, 01, 00, 06, 01, 19, 05, 00, 15, 01, 0a, 0d, 00, 14, 01, 04, 09, 00, 12, 01, 01, 09, 00, 11, 01, 01, 09, 00, 14, 01, 01, 05, 00, 06]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 6
+- Code(Counter(0)) at (prev + 25, 5) to (start + 0, 21)
+- Code(Counter(0)) at (prev + 10, 13) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 4, 9) to (start + 0, 18)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 17)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 20)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 6)
+Highest counter ID seen: c0
+
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::inner_fn
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 1a, 09, 00, 16, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 1e, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 26, 9) to (start + 0, 22)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 30)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::inner_fn_on
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 1f, 09, 00, 19, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 23, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 31, 9) to (start + 0, 25)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 35)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: <auto_derived::MyStruct as auto_derived::MyTrait>::my_assoc_fn::{closure#0}
+Raw bytes (24): 0x[01, 01, 00, 04, 01, 23, 1a, 00, 1b, 01, 01, 0d, 00, 10, 01, 00, 11, 00, 1d, 01, 01, 09, 00, 0a]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 4
+- Code(Counter(0)) at (prev + 35, 26) to (start + 0, 27)
+- Code(Counter(0)) at (prev + 1, 13) to (start + 0, 16)
+- Code(Counter(0)) at (prev + 0, 17) to (start + 0, 29)
+- Code(Counter(0)) at (prev + 1, 9) to (start + 0, 10)
+Highest counter ID seen: c0
+
+Function name: auto_derived::main
+Raw bytes (19): 0x[01, 01, 00, 03, 01, 33, 01, 00, 0a, 01, 01, 05, 00, 1a, 01, 01, 01, 00, 02]
+Number of files: 1
+- file 0 => $DIR/auto-derived.rs
+Number of expressions: 0
+Number of file 0 mappings: 3
+- Code(Counter(0)) at (prev + 51, 1) to (start + 0, 10)
+- Code(Counter(0)) at (prev + 1, 5) to (start + 0, 26)
+- Code(Counter(0)) at (prev + 1, 1) to (start + 0, 2)
+Highest counter ID seen: c0
+
diff --git a/tests/coverage/auto-derived.on.coverage b/tests/coverage/auto-derived.on.coverage
new file mode 100644
index 00000000000..6ab6aa5c4df
--- /dev/null
+++ b/tests/coverage/auto-derived.on.coverage
@@ -0,0 +1,54 @@
+   LL|       |#![feature(coverage_attribute)]
+   LL|       |//@ edition: 2024
+   LL|       |//@ revisions: base auto on
+   LL|       |
+   LL|       |// Tests for how `#[automatically_derived]` affects coverage instrumentation.
+   LL|       |//
+   LL|       |// The actual behaviour is an implementation detail, so this test mostly exists
+   LL|       |// to show when that behaviour has been accidentally or deliberately changed.
+   LL|       |//
+   LL|       |// Revision guide:
+   LL|       |// - base: Test baseline instrumentation behaviour without `#[automatically_derived]`
+   LL|       |// - auto: Test how `#[automatically_derived]` affects instrumentation
+   LL|       |// - on:   Test interaction between auto-derived and `#[coverage(on)]`
+   LL|       |
+   LL|       |struct MyStruct;
+   LL|       |
+   LL|       |trait MyTrait {
+   LL|       |    fn my_assoc_fn();
+   LL|       |}
+   LL|       |
+   LL|       |#[cfg_attr(auto, automatically_derived)]
+   LL|       |#[cfg_attr(on, automatically_derived)]
+   LL|       |#[cfg_attr(on, coverage(on))]
+   LL|       |impl MyTrait for MyStruct {
+   LL|      1|    fn my_assoc_fn() {
+   LL|      1|        fn inner_fn() {
+   LL|      1|            say("in inner fn");
+   LL|      1|        }
+   LL|       |
+   LL|       |        #[coverage(on)]
+   LL|      1|        fn inner_fn_on() {
+   LL|      1|            say("in inner fn (on)");
+   LL|      1|        }
+   LL|       |
+   LL|      1|        let closure = || {
+   LL|      1|            say("in closure");
+   LL|      1|        };
+   LL|       |
+   LL|      1|        closure();
+   LL|      1|        inner_fn();
+   LL|      1|        inner_fn_on();
+   LL|      1|    }
+   LL|       |}
+   LL|       |
+   LL|       |#[coverage(off)]
+   LL|       |#[inline(never)]
+   LL|       |fn say(s: &str) {
+   LL|       |    println!("{s}");
+   LL|       |}
+   LL|       |
+   LL|      1|fn main() {
+   LL|      1|    MyStruct::my_assoc_fn();
+   LL|      1|}
+
diff --git a/tests/coverage/auto-derived.rs b/tests/coverage/auto-derived.rs
new file mode 100644
index 00000000000..cbf8279918a
--- /dev/null
+++ b/tests/coverage/auto-derived.rs
@@ -0,0 +1,53 @@
+#![feature(coverage_attribute)]
+//@ edition: 2024
+//@ revisions: base auto on
+
+// Tests for how `#[automatically_derived]` affects coverage instrumentation.
+//
+// The actual behaviour is an implementation detail, so this test mostly exists
+// to show when that behaviour has been accidentally or deliberately changed.
+//
+// Revision guide:
+// - base: Test baseline instrumentation behaviour without `#[automatically_derived]`
+// - auto: Test how `#[automatically_derived]` affects instrumentation
+// - on:   Test interaction between auto-derived and `#[coverage(on)]`
+
+struct MyStruct;
+
+trait MyTrait {
+    fn my_assoc_fn();
+}
+
+#[cfg_attr(auto, automatically_derived)]
+#[cfg_attr(on, automatically_derived)]
+#[cfg_attr(on, coverage(on))]
+impl MyTrait for MyStruct {
+    fn my_assoc_fn() {
+        fn inner_fn() {
+            say("in inner fn");
+        }
+
+        #[coverage(on)]
+        fn inner_fn_on() {
+            say("in inner fn (on)");
+        }
+
+        let closure = || {
+            say("in closure");
+        };
+
+        closure();
+        inner_fn();
+        inner_fn_on();
+    }
+}
+
+#[coverage(off)]
+#[inline(never)]
+fn say(s: &str) {
+    println!("{s}");
+}
+
+fn main() {
+    MyStruct::my_assoc_fn();
+}